In laboratory tests and the like, highly sensitive and quantitative measurement of a minute amount of a measurement object substance such as protein and DNA in blood would enable quick determination of patient's conditions for treatment. There is therefore a need for a method for quantitatively measuring the measurement object substance in blood with high sensitivity.
Examples of known methods for measuring the measurement object substance in blood with high sensitivity include a surface plasmon resonance (hereinafter abbreviated as “SPR”) method and a surface plasmon-field enhanced fluorescence spectroscopy (hereinafter abbreviated as “SPFS”). These methods utilize a phenomenon in which surface plasmon resonance (SPR) occurs when a metal film is irradiated with light under a certain condition (see, for example, PTL 1).
For example, in SPFS, a capturing body (for example, primary antibody) which can be specifically coupled with a measurement object substance is fixed on a metal film, and a reaction site for specifically capturing the measurement object substance is formed. When a sample (for example, blood) containing the measurement object substance is provided to the reaction site, the measurement object substance is coupled with the reaction site. Next, when the capturing body (for example secondary antibody) labeled with the fluorescence material is provided to the reaction site, the measurement object substance coupled with the reaction site is labeled with the fluorescence material. When the metal film is irradiated with excitation light in that state, the fluorescence material which labels the measurement object substance is excited by the electric field enhanced by SPR, thus emitting fluorescence. Accordingly, the presence or the amount of the measurement object substance can be measured by detecting the fluorescence. In SPFS, a fluorescence material is excited by the electric field enhanced by SPR, and therefore the measurement object substance can be measured with high sensitivity.
Not only in the SPR method and the SPFS but also in methods other than the SPR method and the SPFS, at the time of measurement of a measurement object substance in liquid, the measurement value is normally represented by the mass of the measurement object substance per unit volume of the liquid, a signal amount corresponding to the mass, or the like. Accordingly, when blood is used as a sample, the measurement value is represented by the mass of the measurement object substance per unit volume of the liquid component (plasma or serum) in the blood, the signal amount corresponding to the mass or the like. Since the ratio of the liquid component in blood differs depending on the person, the measurement value of whole blood (blood) cannot be converted to the liquid component measurement value in a uniform manner. In view of this, when whole blood is used as a sample, the hematocrit value of the whole blood (the ratio of the volume of the blood cell in blood) is measured and the measurement value of the whole blood is converted to the measurement value of the liquid component (plasma or serum) with use of the hematocrit value. Examples of conventional methods of measuring the hematocrit value include the micro hematocrit method in which blood is centrifuged, a method of obtaining the hematocrit value from the electric conductivity of blood, and a method of obtaining the hematocrit value from the concentration of hemoglobin of hemolyzed blood (see, for example, PTL 2).